Vanadia coverage

C. Zhao and l.E. Wachs, Selective oxidation of propylene over model supported V2O5 catalysts Influence of surface vanadia coverage and oxide support, J. Catal, 257, 181-189 (2008). 

The TOFs for methanol oxidation to formaldehyde (95-99% selectivity), butane oxidation to maleic anhydride and C0/C02 (30% maleic anhydride selectivity) and S02 oxidation to SO3 are independent of surface vanadia coverage. This observation suggests that these oxidation reactions do not depend on the surface concentration of bridging V-O-V bonds since the reaction TOFs do not correlate with the surface density of bridging V-O-V bonds. Furthermore, the constant TOFs with surface vanadia coverage suggest that only one surface vanadia site is required for the activation of these molecules during the oxidation reactions. 

Information about the number of surface sites required for an oxidation reaction, or activation of the reactant molecule, can be obtained by examination of the variation of the TOF with surface vanadia coverage. In general, reactions requiring only one surface site will exhibit a TOF that is independent of the surface vanadia coverage (surface density of sites) and reactions requiring multiple surface sites will exhibit a TOF that increases with the surface vanadia coverage (surface density of sites). From such an analysis, the number of surface vanadia sites required for various oxidation reactions is presented in Table 3. 

Although UV-vis DR spectra of vanadia on other oxide supports (such as Ti02, Ce02, and Nb205) cannot be readily interpreted because of the overlap of their strong absorptions with those of vanadia, equivalent shifts of the Raman bands as a function of vanadia coverage suggest that the surface VO4 species also polymerize on these supports. 

These conclusions are in line with a recent report that showed TOF values of propane ODH not to change with vanadia coverage on titania or zirconia (Christodoulakis et al., 2004). This Raman investigation corroborated the role of the bridging V-0-MSUpp0rt bond in the kinetically significant reaction steps (Christodoulakis et al., 2004). 

The nature of supported oxides and of the support plays a critical role in the partial oxidation of hydrocarbons since the support is not only providing a high surface area, but also dispersing the oxide. The interaction between the metal oxide overlayer and the imderlying support similarly determines the performance of the catalyst, which may also be affected by the exposed sites of the support. To fully understand these effects, a series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. The monolayer coverage was determined hy Raman spectroscopy and X -ray photoelectron spectroscopy. The activity of the supported vanadium oxide catalysts is determined by the specific support and surface vanadia coverage. 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

The turnover frequencies (TOP), defined as the number of methanol molecules converted to formaldehyde per surface vanadia site per second, are presented for the different supported vanadia catalysts, at monolayer coverages, in Table 2. There is a dramatic variation in the TOFs with the specific oxide support and the variation spans approximately three orders of magnitude at monolayer coverages (the same surface density of surface vanadia species). The TOFs were also relatively independent of surface vanadia coverage... 

It is also important to establish if methanol is directly coordinated to one or two surface vanadia sites, mono-doitate vs. bidentate, or if two surface vanadia sites are required because of lateral interactions among the surface methoxy species at monolay surface vanadia coverage. Comparative IR studies of adsorbed methoxy on vanadia catalysts with known molecular structural reference compounds reveal that the adsorbed methoxy species is only coordinated to one surface vanadia species [20]. This coordination is consistent with the almost constant methanol oxidation TOF as a function of surface vanadia coverage and the insensitivity of the methanol oxidation TOF to the presence of secondary surface metal... 

The molecular characterization studies demonstrated that the oxidized surface vanadia species in the different supported vanadia catalysts possess essentially the same molecular structures predominantly consisting of isolated and polymerized sur ice VO4 species with the same ratio of polymerized to isolated species at comparable surface vanadia coverages. The surface vanadia species even became reduced to comparable extents during methanol oxidation for all the supported vanadia catalysts. The terminal V=0 bond lengths for the different supported vanadia catalysts were 0 essentially identical, see Table 1, and the minor variations in the V=0 bonds did not correlate with Ae methanol oxidation TOFs, (see Tables 1 and 2). Thus, there are no significant molecular structural differences among the surface vanadia species on the different oxide supports to account for the dramatic variation in the TOFs during methanol oxidation over the supported vanadia catalysts. 

The molecular structures of the hydrated surface metal oxides on oxide supports have been determined in recent years with various spectroscopic characterization methods (Raman [34,37,40 3], IR [43], UV-Vis [44,45], solid stateNMR [32,33], and EXAFS/XANES [46-51]). These studies found that the surface metal oxide species possess the same molecular strucmres that are present in aqueous solution at the same net pH values. The effects of vanadia surface coverage and the different oxide supports on the hydrated surface vanadia molecular structures are shown in Table 1.2. As the value of the pH at F ZC of the oxide support decreases, the hydrated surface vanadia species become more polymerized and clustered. Similarly, as the surface vanadia coverage increases, which decreases the net pH at PZC, the hydrated surface vanadia species also become more polymerized and clustered. Consequently, only the value of the net pH at PZC of a given hydrated supported metal oxide system is needed to predict the hydrated molecular structure(s) of the surface metal oxide species. 

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